Optical communication systems are replacing other communication mediums due to several advantages over conventional systems. For example, optical communication systems typically provide wide bandwidth and low attenuation, immunity to electrical noise, and the ability to securely transmit signals over long distances, including transoceanic links. As the networks in these systems grow in size and complexity, however, it has become increasingly difficult yet important to implement efficient routing and wavelength assignment (RWA) algorithms.
The simplest routing algorithm has no knowledge of the network state and selects in advance a path or set of paths to be attempted when service is activated. If the network blocks the setup due to a failure or blocking of a path due to wavelength fragmentation or exhaustion, the next path is attempted until service setup is successful or fails. Network aware routing improves on this approach by flooding network state information through the network (e.g., link up/down or wavelength availability per link). Conventional network aware routing algorithms use this information to preselect from the available paths the paths most likely to succeed. They attempt to avoid paths on which a link has failed, and avoid paths which do not need the requirement for wavelength continuity and have no available wavelengths to carry the service. Typically, routing decisions take place first (sometimes with some knowledge that there is at least one wavelength available), and once a route has been selected, a wavelength assignment algorithm is run to select a wavelength for the service.
There are a variety of conventional approaches to selecting a wavelength, each providing varying levels of success for both the path in question and future paths. Random Assignment, for example, selects a wavelength at random from the available wavelengths on the path. First Fit Assignment selects the first wavelength available in a pre-determined order from the available wavelengths on the path. Close Neighbor Wavelength Assignment selects the wavelength that minimizes gaps between this wavelength and the wavelength in question on neighboring nodes at the start and end of the path. The concept is to prefer a wavelength which has the most neighboring services occupying that wavelength. This attempts to avoid gaps where possible. Most Usage/Least Used Assignment selects the wavelength from the available set which is used the most/least in the network. The advantage of selecting the most-used wavelength is that it attempts to pack more services into the least number of wavelengths, leaving the other wavelengths available for future provisioning or restoration actions. The idea of selecting the least-used wavelength is to spread the service across as many wavelengths as possible to try and maximize the chance of restoration in the event that the transponder or MUX structure of the network is not colorless. Path Length Assignment attempts to separate short paths to one portion of the available wavelengths and long paths to another portion. The concept is that short paths will more easily fit together and be less fragmented, whereas long paths will have more wavelengths available to them without concern for a short path getting in the way and blocking it.
These conventional algorithms have advantages and disadvantages depending on the design of the network. For example, for colorless applications, where wavelengths can be reassigned, the most packing is generally desired to fit more services. For networks with regenerators or wavelength convertors, it may be possible to select a new wavelength at a blocking point to avoid contention at a distant point along the path. When routing takes place independently from wavelength assignment, a network may be analyzed by hand or machine to attempt to determine which algorithm works best for a given network. However, it may be possible to achieve less blocking and better network utilization if routing and wavelength assignment algorithms were to be combined. It may also be advantageous if some algorithms which are path length or neighbor based were to be used in conjunction with other algorithms which use independent criteria for their costs.
Accordingly, there is a need for improved methods, systems, and other techniques for routing and wavelength assignment in optical communications networks.